Magnetic flowmeter for magnetic slurries



April 30, 1968 E, D. MANNHl-:Rz' ETAL 3,380,301

MAGNETIC FLOWMETER VOR MAGNE'HC SLURRIES Filed May ll, 1966 w@ da M5555b b United States Patent O 3,380,301 MAGNETIC FLUWMETER FOR MAGNETCSLURRIES Elmer D. Mannherz, Southampton, and Roy F. Schmooclr, Ivyland,Pa., assignors to Fischer & Porter Co., Warminster, Pa., a corporationof Pennsylvania Continuation-impart of application Ser. No. 350,488,Mar. 9, 1964. This application May 4, 1966, Ser. No. 547,671

8 Claims. (Cl. 731-194) ABSTRACT F THE DSCLSURE A magnetic i'lowmeter tomeasure the flow velocity of magnetic slurries, such as those involvedin the processing of magnetic iron ore, the llowmeter comprising anelectromagnet to establish a magnetic field across a conduit throughwhich the slurry llows, electrodes being disposed in a line extendingperpendicularly with respect to this eld to intercept signals generatedby the flow of slurry through the conduit, a reference coil beinglocated in a portion of the magnetic field to have induced therein areference voltage proportional to the flux through the conduit, therelations-hip between the reference voltage and the signals beingdetermined in order to indicate the flow velocity of the slurry.

This invention relates to magnetic ilowmeters for measurement of flow ofmagnetic slurries such as those involved in the processing of magneticiron ores, for eX- ample those of the taconite formations of the Mesabirange. The invention is applicable to magnetic slurries generally and isalso concerned with the measurement of the concentration of magneticmaterial in such a slurry. This application is a continuation-impart ofthe copending application entitled i ragnetic Flowmeter, Ser. No.350,488, filed Mar. 9, 1964, now abandoned.

Conventional magnetic owmeters are not suitable for measuring the flowof magnetic slurries, Typically, as used for the measurement of what arefrom the practical standpoint non-magnetic fluids, such a flowmeterinvolves an exciting coil wound on a core struct-ure providing magneticflux transverse to a conduit through which liquid flow is occurring andhas a reference voltage provided either by a transformer having itsprimary connected in series Awith the exciting coil or by a coil woundon the same core as the exciting coil and threaded essentially by thetotal ux through the core.

When the liquid is substantially non-magnetic, either such source ofreference voltage is thoroughly satisfactory for comparison with thepotential provided at the pickup electrodes. These electrodes areusually arranged on a line mutually transverse to the directions of themagnetic eld through the flow conduit and the liquid flow. ln each casethe reference voltage is proportional to the magnetic flux, thegeometric pattern of which is essentially the same for all flow rates ofall non-magnetic liquids. Desirably, the magnetic field throughout thecross-section of the conduit is uniform, and to achieve this aconfiguration is used in which only a part of the flux threads theconduit, substantial other parts thereof passing outside the conduit.Since the effective reluctance of the conduit and the liquid therein isapproximately that of air, t-he geometry of the magnetic field remainsconstant under all conditions of operation.

But when the flowing nid consisting of the liquid whose suspendedcontent of solid magnetic material is highly magnetic and of varyingcomposition, such references are unusable. The presence of the fio-wingmagice netic material lowers very considerably the reluctance of the'gapthrough which flow occurs between the pole faces of the core structure,so that the geometry of the magnetic eld changes considerably with theconcentration of the magnetic material and the ratio of the flux densitythreading the conduit to the exciting current changes correspondingly.In -other words, the flux density threading the liquid, and which isresponsible for the ouput signals, is no longer proportional either tothe current through the exciting soil or to the total flux which threadsthe core.

Accordingly, it is the main object of this invention to achievesatisfactory metering of a magnetic slurry by deriving a referencevoltage from a coil so positioned and dimensioned as to be threaded onlyby a portion of the flux which threads the conduit through which theslurry flows, so as to be proportional to that part of the total -uxwhich is responsible for production of the output potential at theelectrodes. Thus the effects of changes in the geometric configurationof the magnetic field due to changes in the reluctance of the fluid areminimized.

Also an object of this invention is to measure the concentration of themagnetic slurry. The reference voltage itself is a function of thechange of reluctance and, therefore, of the concentration of themagnetic material in the -tluid independent of velocity, so that thereference voltage may be `used as a measure of this concentration. Thefunctional relationship between reference voltage and concentration ofmagnetic material is obtained by calibration.

It is somewhat complex in its derivation. For a given supply voltage tothe exciting coil, the decrease in 4reluctance across the gap due toincrease in amount of magnetic material therein produces an increase inself-inductance of the coil, and, accordingly, less exciting currentflow; but the decreased reluctance increases the proportion of the totalflux which threads the conduit, and accordingly increases the ux throughthe reference coil so that the reference voltage will increase withincrease in concentration of the magnetic material.

Still another object of the invention is to provide a flowmeter of theabove type for measuring magnetic slurries with a high degree ofaccuracy, the meter being substantially insensitive to changes intemperature.

For a better understanding of the invention, as well as other objectsand further features thereof, reference is made to the `followingdetailed description to be read in conjunction with the accompanyingdrawing, wherein:

FIG. 1 is a diagrammatic showing of the structural and electricalcomponents of a flowmeter for measuring magnetic slurries, in accordancewith the invention, and

FIG. 2 is another embodiment of a flowmeter in accordance with theinvention, which is insensitive to changes in temperature, only thatportion of the system being shown which is at variance with the systemshown in FIG. l.

The structure and circuit of the flowmeter The ilowmeter is similar inmany respects to that shown in Kass Patent 3,094,000 dated June 18, 1963and it comprises primary and secondary units. Reference may be made tothe Kass patent for structural aspects and also the electrical elementsand their operations which are not specially changed in accordance withthe present invention. However, the flowmeter shown in the Kass patentis of the type used for the measurement of ow of nonmagnetic fluids.

Refering now .to FIG. 1, the meter comprises a tube 2 for carrying themagnetic slurry being measured. This tube 2 may be of metal, such asstainless steel, lined with an insulator such as neoprene, or it may beformed of an insulating plastic. At diametrically opposite points in thetube 2, inwardly extending electrodes 12 are provided,

which are exposed to the liquid. The electrodes are insulated from thewall of the tube if this is conductive.

A magnetic field of uniform type is established which extends at rightangles to the diameter forming the common axis of the electrodes 12 andto the tube axis, this lield being produced by a pair of coils 22,preferably of the form described in said Kass patent. A magnetic core 24is associated with these coils and presents pole faces 25 to theconduit.

As is usual, the entire flux between pole faces 25 does not thread theconduit. This situation is intentionally adopted in order that themagnetic field throughout the cross-section of the conduit will beessentially uniform, which would not be the case if the fringe fluxlines passed through the conduit. 1t is for the reasons just indicatedthat variation of reluctance of the material within the conduit willproduce change of distribution of the flux, particularly as between thatwhich threads the conduit and that which does not. However, `the fluxdistribution through the conduit itself will remain substantiallyuniform, though its intensity will vary.

For the purpose of providing the reference voltage, a coil 26 isprovided with its axis located centrally of the magnetic field and theconduit. This coil may be provided above or below the conduit asindicated. It may have any suitable number of turns, but its diameter isdesirably limited so that it will be threaded only by a portion of theflux passing through the conduit. This is in order to have it threadedsolely by the uniform fiux passing through the conduit and not by anyfringe flux which may vary in configuration in dependence upon thereluctance of the mixture undergoing measurement. The dimensions are, ofcourse, not critical, but desirably the coil does not have a diametersubstantially exceeding about 1/3 the diameter of the conduit and it maybe much smaller than this. Considering placement along the axis of theconduit, it should be in the central portion of the extent of the field.

Alternating current is supplied from the terminals 50 connected to theusual power supply, for example 110 volts at 60 cycles. FIG. 1 alsoshows various direct current supply terminals and it will be understoodthat these are fed by conventional direct power supplies energized fromthe commercial alternating supply.

The magnetic field windings 22 are connected in series or parallel andto the supply terminals 50. The coil 26 is connected to a networkcomprising the capacitor 58 connected across the coil and the parallelresistance arrangement comprising in series the fixed and adjustableresistors 60 and 62 and the adjustable resistor 64, to the terminals ofthe latter there being connected the leads 66. The leads 66 feed anamplifier 67 to raise the signal applied thereto to a suitable level,the output leads from this amplifier being shown at 69.

Amplification in this fashion is not required in accordance with thedisclosure of the Kass patent since the reference voltage may be derivedfrom a transformer connected in the supply lines to the field coils. Inthe present case, however, the coil 26 is in a magnetic circuit whichhas a considerable gap so that, unless a great many turns are used theinduced voltage will be low and there will not be good matching to theremaining parts of the circuit. The amplifier 67 therefore provides bothamplitication and proper matching of impedances.

A meter 71 connected across the lines 69 is used for measurement of theconcentration of magnetic material in the flow. This meter may, ofcourse, be of indicating or recording type.

While the elements of the network are interdependent, the adjustment ofresistor 62 primarily affords phase adjustment while that of resistor 64affords amplitude adjustment. These afford corrections for eddy currentshifts. The result of the adjustments is to provide a constant ratiobetween the potential per unit velocity appearing at the electrodes andthe current which is provided at the conductors 66. The ultimate resultis that the response of the secondary unit is full scale in terms offeet per second of liquid iiow velocity for any primary unit which maybe associated with a secondary unit.

The leads from electrodes 12 are connected individually through thesecondaries 68 and 70 of identical transformers 72 and 74, and throughcapacitors 76 and 78 to the grids of triodes 8i) and 82. The primariesof the transformers 72 and 74 are connected in parallel between groundat 84 and a line 86 in such fashion that signals fed back through theline 86 will null the signals from the electrodes, the connections beingsuch that opposition to the electrode potential is provided by eachtransformer. The symmetrical arrangement here adopted involves rejectionof signals which may ow in the same direction through the symmetricalconnections.

The feedback signal flowing through line 86 is derived from a networkreceiving its input from lines 69. A potentiometer 88 connected betweenthese lines has its adjustable contact 90 grounded. A secondpotentiometer 92 is connected between these lines to provide a variableresistance. A third potentiometer 94 connected between these lines hasits adjustable Contact 96 connected through capacitor 9S to theconnection 86. A fourth potentiometer 100 connected between lines 69 isarranged, as illustrated, with its variable contact 102 connected to oneof the lines through a resistor 104 and through a variable resistor 106and a fixed resistor 108 to the range adjustment network, generallyindicated at 110.

This network comprises a group of ganged switches 112, 114, 116, 118 and120, connected as illustrated between the resistor 108 and ground inconjunction with the equal resistors 122 and 124 having fixed values.The series arrangement of fixed resistor 12S and adjustable resistor126, and the potentiometer 130, the adjustable contact 132 of which isconnected through resistor 134 to the resistor 168 and through resistor136 to the connection 86, provides an output to the connection 86. Apair of small capacitances are connected in parallel between theconnection 86 and ground` As will more fully appear, the potentiometercontact 102 is adjusted by a reversible motor 18S. The functions of thevarious parts of the network just described are as follows:

Potentiometer 88 serves as an electrical centering control to set zeroflow at any desired position on the recording chart of the meter. Thismakes it possible to indicate the measure bidirectional flow where thatis required. The nature of this action will be evident upon consideringthe ground connections of contact 90 and, at 84, the ground connectionof the primaries of transformers 72 and 74.

Adjustable resistance 92 acts to set the input resistance of thebalancing network. This input resistance is desirably of low value,typically, for example, about 81 ohms, and by the use of the adjustmentunder discussion the input resistance may be set to such a value thatvarious secondary units may be made interchangeable.

Potentiometer 94 and its connection through capacitor 9S provides a nullcontrol allowing an operator to null out unwanted signals which are inquadrature with the error signal and aids in phasing the servo amplifierprecisely, with greater accuracy than is attained by using anoscilloscope. The proper phase of quadrature signal is obtained by useof the capacitor 98 and reactance of which is many times that of thetotal network. A phase shift obtained from this capacitor is very nearly90 and the shift gives essentially a true quadrature signal.

Balancing is effected by the motor controlled movements of the contact102 of potentiometer 196 associated with the fixed resistor 104 whichcompensates for the load on the potentiometer lit() caused by the rangeadjustment network, and with the adjustable resistance 106 whichcompensates for the loading of the range network by the input impedanceof the balanced transformers 72 and 74.

The balance signal is fed and attenuated through the range adjustingnetwork so that full scale sensitivity is accurately known. Adjustableresistor 126 serves for trimming. The range potentiometer 130 isdesirably of multi-turn type and constitutes in conjunction withresistors 122 and 124 a voltage divider network. With resistors 122 and124 equal (for example, having values of 450 ohms each) and with theparallel arrangement of potentiometer 130 and the adjustable and fiXedresistors in parallel therewith providing an effective resistance of thesame value (potentiometer 130 having, for example, a resistance of 500ohms), the switching arrangement is such as to locate the potentiometerin any one of three alternate positions in a series arrangementincluding it and the resistors 122 and 124. Thus, considering anarbitrary over-all range of 0 to 30, the placement of the potentiometerresistance may be in a range 0 to 10, 10 to 20, 20 to 30, depending onthe position of the switches, so that full range adjustment of thepotentiometer may occur throughout any of these ranges.

The inductive reactance of balancing transformers 72 and 74 causes aphase shift of the balancing signal which must be corrected, and thiscorrection is obtained through the use of capacitors 13S which may bechosen to suit particular units since the necessary correction variesfrom unit to unit. Through the use of standard capacitors, one beingrelatively large and the other being small to act as a trimmer, it isunnecessary to provide an adjustable capacitor for this phasecorrection.

If it were assumed that there was an .indicator of the potentialdifference between the grids of triodes 80 and 82, and if adjustment ofthe contact 102 of potentiometer 100 was made to provide a zeropotential difference at these grids, i.e., a null, it will be evidentthat the setting of the potentiometer contact would be a measure of theliquid fiow. The manner in which automatic adjustment is achieved `tosecure a null will now be described.

Triodes 80 and 82 and their associated circuits constitute apreamplifier for the net output from the secondaries of transformers 72and 74 and the electrodes 12. In this connection it may be noted thatthese transformers are preferably located in the primary unit assemblyto reduce the effect of cable capacitance as a shunt of signals whichoriginate in high resistance liquid. In Stich case, capacitors 138should also be in the primary unit, for they correct for the phase shiftdue to the inductive reactance of the transformers. A cable connection-may thus either precede or follow these transformer secondaries fortheir connection to the remaining portions of the circuit.

The preamplifier primarily affords an impedance matching device andtransformation from a balance to-unbalance arrangement. The triodes areconnected in pushpull arrangement to the primary windings 140 of atransformer 142, the secondary 144 of which feeds amplified signalsthrough a transformer 159 to the first stage triode 152 of the mainamplifier. Special filtering is provided by network 148 for the positivesupply provided to the triodes 80 and 82 from a positive supply terminal146 of the power supply.

The main ampliher includes the triodes 152, 154, 156 and 164 ingenerally conventional cascade stage form. Phase shift adjustment iseffected by variation of contact 153 of potentiometer 151, providing avariable resistance associated with capacitor 155, gain control beingprovided by potentiometer 158. In order to avoid hum it is desirable toprovide to the heaters of triodes S0, 82, 152 and 154 suitable directcurrent which may be derived from the supply through a suitablerectifier and simple filter system, not shown. Rate feedback control isprovided at the potentiometer in the cathode-to-ground return of triode156.

In order to provide sufiicient motor driving power,

a pair of triodes 166 and 168 in parallel arrangement provide a poweramplifier. Their output is fed through resictor 17!) to the fieldwinding 172 of motor 188. The other phase winding 186 of this motor isprovided with reference current from the terminals 50 through capacitor187. It will be understood that the motor is of a type which reverses inaccordance with the phase relationship of the currents thro-ugh itswindings 172 and 186, remaining stationary when the current in winding172 is in quadrature with that properly produced therein by desiredsignals picked up by electrodes 12. Shunted aero-ss held winding 172 isthe series arrangement of an alternating current voltmeter 173 and acapacitor 175.

The primary 174 of a transformer 176 is connected between the signaloutput side of resistor and the parallel arrangement of resistor 186 andcapacitor 178, the right-hand end of the transformer primary 174 beingconnected through resistor 182 to a positive supply terminal which maybe the same terminal as that to which the winding 172 is connected. Thesecondary of the transformer 176 provides a signal between ground andthe adjustable contact of the rate adjustment potentiometer 160, theconnection being through resistor 184 and lead 162. This rate feedbackcontrol has its usual functions.

In a fiowmeter of this type hydraulic noise may cause rapid excursionsof a recording pen producing a broad line on the chart and this isundesirable. Heretofore, these excursions have been damped out by theuse of dashpots, but they, in turn, greatly slow down the response. Inthe present system provision is made electrically for an action whichcorresponds, roughly, to the use of backlash in mechanical gearing butwith provision for proper adjustment. In brief, this is accomplished byoperating the final amplifier stage comprising the triodes 166 and 16%under class C conditions thereby limiting the response to large signalexcursions only, in excess of those which would be due to noise Tosecure this result, variable bias is applied to the grids of triodes 166and 16S. A contact 192 of potentiometer 194 is driven by motor 188 toeffect automatic adjustment. Potentiometer 194 is connected in serieswith a resistor 196 between ground and a negative bias supply terminal198 of the power supply. Also between this terminal 193 and ground areresistor 200 and potentiometer 202, the adjustable Contact 204 of whichis connected through resistor 206 to the grids of the final stagetriodes 166 and 168. The potentiometer contact 192 is joined to thejunction of resistor 2610 and potentiometer 202. The network justdescribed is provided because hydraulic noise is not preciselyproportional to liow rate.

Resistance network comprising 194, 196 and 200 provides an outputvoitage across potentiometer 202 so that the smoothing control thusconstituted has the desired voltage characteiistic. Potentiometer 202has a high resistance as compared with the other resistances in thissmoothing network and consequently does not alter the characteristic ofthe output voltage but is a manually adjustable amplitude control usedonly to limit the bias voltage as dictated by the hydraulic noise of thesystem. The ganging of contact 192 with contact 102 determines therelative amount of smoothing voltage applied as a bias to the amplifierstage.

Because of the application of the negative bias varied in accordancewith desired operation, the last amplifier stage operates under class Cconditions so that signal eX- cursions less than a predeterminedamplitude do not produce motor-driving output` The amount of the biasdetermines the minimum signals received from triode stage 164 which willproduce motor drive. The amount of this minimum may be manually adjustedthrough potentiometer contact 264, while the amount is alsoautomatically adjusted by the operation of motor 1S?.- the range of thedead region within which the drive will not be effected varying with theflow rate as reflected by the position of,

desirably greater for larger flow rates than for smaller fiow rates.Further, it is desirable for the width of the dead region to becomeesssentially Zero, as a result of operation of contact 192, when theflow is somewhat greater than zero, to assure a live zero, and theconstants of the circuit are chosen accordingly.

While element 188 has been generally referred to as a motor, it will beunderstood that in practice this may be a conventional phase-sensitiverecorder ,motor driving through reduction gearing, the potentiometercontacts 102 and 192 and either an indicator or a marking pencooperating with either a fixed or time driven chart scale indicated at199. 1n conventional fashion this may also operate controls related tothe flow, eg., to maintain the flow constant, to effect other operationsin accordance with the flow, or the like.

T he opemlion of the flou/meter The overall operation of the iiowmeterillustrated in FlG. 1 may now be briefly outlined as follows:

For a given rate of flow through tube 2, there will be produced anoutput voltage across electrodes 12 the magnitude of which isproportional to the flow rate for a given magnetic field strengththreading the conduit and produced by windings 22. Prior to balance,corresponding signals are applied to the amplifier system to furnishcurrent to motor winding 172 which will drive motor 188 and with it thepotentiometer Contact 102 to provide a feedback signal to balance theelectrode signal to supply a Zero input to the amplifier. In case of avoltage chan-ge at terminals 50 affecting the strength of the magneticfield, a corresponding change in output from transformer 54 occurs so asto balance out effectively such variations. Adjustments which have`already been described take care of quadrature potentials which enterinto the system.

The foregoing assumes error signals of sufiicient magnitude to drivemotor 188. The smoothing arrangement, providing bias to the last stageof the amplifier, prevents such movements when the error signals due tonoise fluctuations are insufficient to provide output from the class Camplifier stage. Despite the fact that small fiuctuations will not giverise to response of the motor, it should be noted that class C operationreferred to does not involve any deterioration of response to signalsexceeding those which are to be effectively suppressed. Thus there is noloss in rapidity of response to desired signals.

The use of meter 173 for the adjustment may now be described.Potentiometer 94 serves to introduce a quadrature signal to the -line86. The introduction of a quadrature potential amplified and deliveredto motor Winding 172 (as Well as to the voltmeter 173) should produce norotation of the motor, i.e., no change in flow indication, if thecorrect phase relation between the amplifier input and the motor fieldcurrent exists.

A test for proper amplifier phase adjustment -may be made by manuallymoving potentiometer contact 96 in both directions so that voltmeter 173reveals lamplifier quadrature signals. As -a result of such signalchanges there should be no change in fiow indication. If such changeoccurs, it is necessary to adjust the phase of the amplifier output bychange of the setting of contact 153 of potentiometer 151.

It will be apparent that when the quadrature signal, which is always at90 with respect to the flow signal, produces no defiection of the flowindicator, the phase relationship between the amplifier input and theservomotor field will -be that for maximum sensitivity of the flowsignal and for optimum rejection of any spurious quadrature signal thatmay arise in the flow detector. When the phase of the `amplifier hasbeen proper-ly adjusted by contact 153 so that movement of potentiometercontact 95 and correspondingly large variations in readings on meter 173are no longer accompanied by changes in flow indication, contact 96 isthen adjusted to give the minimum achievable reading on meter 173, thesignificance of which minimum reading would be that minimum quadraturesignals were being introduced to the amplifier.

Small phase drifts with time in the amplifier can then produce nosignificant error in fiow indication in the absence of substantialquadrature signals. Conversely, if, as would be unlikely, a spuriousquadrature signal appeared over a long time period, no error of flowindication would result so long as the phase situation remainedsatisfactory. Only in the event of the extremely unlikely simultaneousoccurrence of a large spurious quadrature signal and of a considerableamplifier phase drift would there occur an error in flow indication, anda large reading on the meter 173 would make this situation immediatelyobvious.

It is also to be noted that lines 69 receive from coil 26 an alternatingsignal which ma' be considered the inphase signal, preliminaryadjustments having been made to achieve this condition as closely aspossible with respect to the electrode signals. Starting, therefore,with the signal across lines 69, an alternating signal through thefeedback network running from 88 through 108 and from 108 through rangeadjustment network 110' to line 86 is provided, delivered to theprimaries of the transformers 72 and 74.

Noting that only resistances are involved in this path (neglecting thehigh reactance at 98) the signal is an inphase one. Across theresistance of potentiometer 94 there is, then, an in-phase potentialwith respect to ground represented by the contact 90 of potentiometer88. Accordingly, an in-phase potential appears lat contact 96. Aspreviously pointed out, the reactance of capacitor 98 is many times thatof the total resistances involved in the network. Therefore, the currentthrough 96 is 90 out of phase, so that the signal component thusprovided through capacitor 98 to the primaries of the transformers 72and 74 is a quadrature one. The magnitude of this signal may be variedby movement of contact 96.

The foregoing explains the quadrature input to line 86. If it were notfor the slight corrections between the secondary 56 of transformer 54and the lines 69, the quadrature potential could be applied to the line86 directly from the supply terminals 50, utilizing a high reactancecapacitor. However, it is desirable to use as the source for thisquadrature signal a portion of the system which carries an in-phasesignal already corrected.

In contradistinction from what is disclosed in the Kass patent, it willnow be evident that the reference voltage is derived from the coil 26and is a measure solely of the flux -which passes through the conduitand is not a measure of either the current to the exciting windings 22,nor the total flux passing through the core only some of which threadsthe conduit. In this regard it may be informative to cite typicalfigures obtained, utilizing the fiowrneter as just described.

With water flowing through the meter With no content of magneticmaterial, the magnet current, through the windings 22, was 1.37 amperesand the reference voltage appearing across lines 69 was 4.23 volts. Ther-atio of signal volta-ge overflow rate was found to be 1.835 millivoltsper gallon per minute. This gave a signal voltage per flow rate perreference voltage equal to 0.434.

In contrast, when there was fiowing a slurry containing 32.5 ofmagnetite by Weight, the magnet current dropped to 1.27 amperes whilethe reference voltage across lines 69 rose to 4.95 volts. The ratio ofsignal voltage over fiow rate was then 2.175 millivolts per gallon perminute. The signal voltage per fiow rate per reference voltage was0.439.

From the foregoing it will be seen that the signal voltage per fiow rateper reference voltage remained very nearly constant with an error ofonly 1.24 which figure represents a quite satisfactory measure ofindependence of the flow measurement with respect to the contents ofmagnetic material. The -drop of magnet current with the presence ofmagnetic material indicates the change of inductance produced by the lowreluctance of the magnetic path. The increase in reference voltage, onthe other hand is indicative of the major change of geometry of theiiux, indicating the greater threading of the conduit by the field.

The change of reference voltage may be used as a measure of theconcentration of magnetic material in the fiowing liquid, meter 71serving for this measurement. As pointed out previously, because thechange in reference voltage with respect `to concentration of magneticmaterial is derived from a rather complex set of circumstances, thepractical aspects of this measurement involve calibration of thefiowmeter using known concentrations of the magnetic material. Suchcalibration may -be readily made and the results are substantiallyindependent of adjustments which may be made in, or operations of, thefiow measuring portions of the circuit. It need only be insured that thecommercial supply voltage is reasonably constant.

Temperature compensated fi'ow/'neter A system of the type disclosed inFIG. 1 has some degree of sensitivity to variations in temperature, andwhere the system is installed in an environment subject tto largetemperature changes, it is desirable that the system be renderedsubstantially insensitive to such changes.

In the system shown in FIG. l, the output of the reference voltage coil26, which lies in the air gap, is a function both of the level of thefiux density and the distribution of ux in the vicinity of the coil. Ina system in which the reference is -derived from the line voltage, anychange of copper resistance, eddy current losses, or inductance of thecoils, `due to geometry variations of iron permeability variations, willaffect the relationship between the total flux and line voltage andtherefore cause a change in indication. These factors are all subject tovariation with changes in temperature. A similar situation exists whenusing a current-derived reference, except that the effects due to copperresistance changes are obviated.

In order, therefore, to render the system insensitive to temperaturevariations, it is essential that the reference signal be deriveddirectly from the total fiux rather than to infer a function of totalflux density level from some other parameter. In this way, outputvariations .due to changes in eddy current losses and other factorsmentioned above, may be eliminated or minimized.

To accomplish this purpose, a coil 205, hereinafter referred `to as atotal liux coil, is wound about the outside diameter of one of the twofield magnet coils 22. In practice, the coil may consist of nine turnswound around the outside diameter of one of the magnet coils and tapedrigidly thereto. The output of the total flux coil 205 is phase-shiftedby a network including variable resistor 206 in series with capacitor207. The output of the network is derived from the junction of theresistancecapacitance circuit and the junction of two series-connectedresistors 208 and 209, shunted across the total fiux coil. This output,which is a correction voltage, appears across terminals 210 and isapplied in series with the reference voltage output of the coil 26, thelatter being attenuated by a resistance network including potentiometer211. The corrected or net reference output is available at terminals212, and is fed to amplifier 67 and thereafter processed in the mannerdescribed in FIG. l with the reference voltage.

The output of total fiux coil 205 is phase-shifted and attenuatedrelative to the output of reference voltage coil 26 so that with nomagnetite or other magnetic rnaterial present in the fiowmeter conduit,its output is equal and opposite to the signal from the referencevoltage coil. Any change in total flux will affect both coils in thesame manner, hence the net output signal, which is zero withoutmagnetite, does not change. With magnetite present in the meter, nochange results in the total fiux, but a change 10 takes place in thedirection of the fiux in the air gap. This change in direction givesrise to a change in the output from the reference voltage coil in theair gap, and therefore the net signal from the two coils, this netsignal being a function of the percentage of magnetite.

Inasmuch as total fiux coil 205 has an output which in practice is lowerthan that from reference voltage coil 26, it is the reference coilsignal which is attenuated rather than the total fiux coil signal toattain zero output in the absence of magnetite. A further attenuation ofthe net signal is generally necessary to prevent the signal withmagnetite present from exceeding the range of the secondary.

It has been found that when using a system of the type ldisclosed inFIG. 1, the temperature effect was as high as 1% per 10 F. change intemperature, whereas `when the system was modified in the mannerdisclosed in connection with FIG. 2, the temperature effect for the same10 F. change was .04% magnetite shift. In both instances, a -three-inchmagnetite primary was used and a known amount of magnetite, only theprimary of the flowmeter being subjected to varying temperature in therange of F. to 150 F. Thus a 25-to-1 improvement was attained.

While there has been shown and described preferred embodiments of myinvention, it will be appreciated that many changes and modificationsmay be made therein without, however, departing from the essentialspirit of the invention as defined in the annexed claims.

What I claim is:

1. A magnetic fiowmeter comprising,

(a) a conduit for iiowing a magnetic slurry,

(b) electromagnetic means establishing a magnetic field extendingtransversely through said conduit, said means including pole faces whichare presented to said conduit and extend therebeyond to produce fluxlines which are substantially uniform throughout the cross-section ofsaid conduit, the fringes of said fiuX lines being outside of saidconduit,

(c) electrodes exposed to said magnetic slurry fiowing through theconduit and disposed in a line extending perpendicularly with respect tosaid magnetic field to intercept signals generated by the fiow of slurrythere` through, and

(d) a reference coil located in and threaded by a portion of said field,said coil being restricted to a portion of said field which passeswholly through said conduit and having induced there in a referencevoltage proportional to the iiux through the conduit, and means todetermine the relationship of said reference voltage to said signals toproduce an indication of the fiow velocity of said magnetic slurry.

2. A magnetic fiowmeter, as set forth in claim 1, wherein saidlast-named means includes means responsive to said reference voltage toprovide a voltage bucking said signals, and means providing anindication of the relationship of the bucking voltage and said signals.

3. A magnetic fiowmeter, as set forth in claim 1, further includingmeans to measure said reference voltage to afford an indication of theconcentration of the magnetic material in said slurry.

4. A magnetic fiowmeter, as set forth in claim 1, further includingmeans supplying an alternating current t0 said electromagnetic means.

5. A magnetic fiowmeter, as set forth in claim 1, further including acoil surrounding said electromagnetic means to produce a correctionvoltage representing total fiux, and means to apply said correctionvoltage in series with the reference voltage to produce a net referencevoltage to compensate for temperature variations.

6. A magnetic fiowmeter comprising,

(a) a conduit for liowing a magnetic slurry,

(b) an electromagnetic coil disposed with respect to said conduit toestablish therein a magnetic field extending transversely therethrough,

(c) electrodes exposed to said magnetic slurry owing through the conduitand disposed on a line extending perpendicularly with respect to saidlield to intercept signals generated by the ow of slurry therethrough,

(d) a reference coil located in and threaded by a p0rtion of said field,said coil being restricted to a por tion of said field which passeswholly through said conduit and having induced therein a referencevoltage proportional to the flux through said conduit,

(e) means responsive to said reference voltage to produce a voltagebucking said signals,

(f) means providing an indication of the relationship of said buckingvoltage and said signals to indicate ow velocity, and

(g) means to measure said reference voltage to indicate theconcentration of magnetic material in said slurry.

7. A owmeter as set forth in claim 6, further including a total fluxcoil surrounding said electromagnet means to produce a correctionvoltage representative of total ux, and means to apply said correctionvoltage in series with said reference voltage to produce a net referencevoltage to compensate for temperature variations.

8. A owmeter as set forth in claim 7, further including a phase-shiftingand attenuating network coupled to said total tlux coil to adjust thelevel of said correction voltage relative to the level of said referencevoltage to produce a net reference voltage which is zero in the absenceof magnetic material in said conduit.

References Cited FOREIGN PATENTS 834,011 5/196() Great Britain.

I AMES J. GILL, Primary Examiner.

C. A. RUEHL, Assistant Examiner.

